This lecture covers the principles of solar energy conversion devices and plants, focusing on electrochemistry, thermodynamics, and efficiency calculations. Topics include the working age of fuel cells, reduction potential, overpotential components, mass transfer modes, and potential step methods under diffusion control. The lecture also delves into the Butler-Volmer equation, current-voltage curves, and the analytical expression for current-voltage plots. Emphasis is placed on understanding the relationship between current, heat, and power in solar energy systems.